DC-057: close checkout-to-license contract drift (grade B)

Canonical product catalog at src/billing/catalog.js shared by Stripe
Checkout client (src/billing/stripe-client.js), webhook bridge
(scripts/stripe-license-bridge.js), and pricing page
(status/pricing/index.html). One-time payment keyed by productId at
$20/$50/$70/$99 — no more monthly/annual subscription drift.

Bridge resolves duration via metadata.productId (single contract),
requires payment_status === 'paid' before fulfillment (rejects
unpaid/no_payment_required/missing with ack 200), handles
async_payment_succeeded for ACH/SEPA delayed-payment flow. License
persisted to fulfillment-store BEFORE email — SMTP failure path serves
the persisted code via the new /api/v1/billing/lookup/:sessionId
endpoint (the documented customer recovery path).

Layer-1 (event-id) + layer-2 (session-id) idempotency prevent
duplicate issuance. Checkout return URLs derived from
STRIPE_PUBLIC_ORIGIN or STRIPE_ALLOWED_HOSTS (not raw Host header) —
closes host-header-poisoning + session-ID-leak attack class.

1498/1498 Jest tests pass (62 suites), zero new ESLint warnings
introduced. Test files:
  - stripe-license-bridge.test.js (24 tests)
  - billing-lookup.test.js (8 tests, HTTP-level)
  - bridge-lookup-http.test.js (5 tests, uses exported createServer)
  - pricing-page-catalog.test.js (9 tests, per-tier consistency)
  - checkout-origin.test.js (6 tests, host injection rejection)
  - stripe-client.test.js (rewrite for productId + mode:payment)

Bridge code refactored: handleWebhook decomposed into verifySignature +
parseEventBody + checkEventIdempotency + fulfillCheckout +
ensureLicensePersisted (under ESLint complexity=20 cap). New
createServer()/createRequestHandler() factories guarded by
require.main === module.

Removed 3 stale test files from the rolled-back DC-055 attempt.

dashcaddy-api/src/app.js

@@ -85,6 +85,7 @@ const eventsRoutes = require('../routes/events');
 const workflowsRoutes = require('../routes/workflows');
 const dependenciesRoutes = require('../routes/dependencies');
 const securityRoutes = require('../routes/security');
+const billingRoutes = require('../routes/billing');
 const DependencyManager = require('./managers/dependency-manager');
 const autoRestartRoutes = require('../routes/auto-restart');
 const configDriftRoutes = require('../routes/config-drift');
@@ -528,6 +529,11 @@ async function createApp() {
     asyncHandler: ctx.asyncHandler,
     log: ctx.log,
   }));
+  // DC-055: billing is PUBLIC (customer hasn't paid yet → no session).
+  // Stripe-session creation only; the webhook side runs in scripts/stripe-license-bridge.js.
+  apiRouter.use('/billing', billingRoutes({
+    asyncHandler: ctx.asyncHandler,
+  }));
   apiRouter.use('/dns', dnsRoutes({
     dns: ctx.dns,
     siteConfig: ctx.siteConfig,